1. Field of the Invention
This invention relates to multilayer films, and, more particularly, to tear resistant multilayer films comprising alternating layers of relatively stiff and ductile polymeric materials.
2. Description of the Related Art
Traditionally, "tear resistance" has described the ability of a film to resist continuing to tear once a tear has been started. Trash and grocery bags, often based on polyolefins such as polyethylene, are examples of films that are conventionally considered to be tear resistant. These films have considerable stretch which enables them to resist advancing an already formed tear. By "stretch" it is meant that the films have a low tensile modulus and are not dimensionally stable.
Also known are films which are relatively stiff. In this regard, "stiff" refers to films which cannot be stretched significantly without breaking; that is, films which are dimensionally stable, creep-resistant (stretch resistant), and of high modulus. Examples of stiff, dimensionally stable, high modulus materials are certain packaging films such as cellophane, polyesters and biaxially oriented polypropylene. However, these films have low tear resistance. That is, once a tear has been started, the film continues to tear quite easily.
There are numerous applications where stiff, tear resistant films would be desirable. For example, films which provide sign faces and building awnings must be tear resistant to have a useful life. On the other hand, these films must also be relatively stiff so that they will not billow in the wind or sag with age.
Backings for abrasive sanding belts experience harsh operating conditions and must resist tearing. However, sanding belts which stretch are undesirable because they may not fit securely on the sander and may work free under normal use.
Angioplasty balloons for expanding blood vessels during surgery require stiff, tear resistant films. The balloons cannot readily shatter (i.e., tear) during use. The balloons must also inflate to a controlled size and should not stretch to a larger size.
For certain tapes, stiff, tear resistant backings would be desirable. Such backings would not readily continue to tear if inadvertently nicked or cut when dispensed. At the same time, the backings would be stretch resistant which could enhance the stability of articles taped therewith.
Films for shatterproofing windows need to be tear resistant. However, the performance of such films would be enhanced if the films were also stiff and tear resistant as the combination of these properties would help the film to absorb energy in the event of a window shattering impact.
Numerous packaging films are disclosed in the prior art. U.S. Pat. No. 3,188,265, "Packaging Films" issued Jun. 8, 1965 to R. Charbonneau, et al. discloses a heat-sealable film comprising polyethylene extruded onto a web of oriented polyethylene terephthalate. U.S. Pat. No. 4,705,707 "Polyethylene/Polyester Non Oriented Heat Sealable, Moisture Barrier Film and Bag," issued Nov. 10, 1987 to J. Winter discloses a moisture barrier film useful in microwaveable food pouches. The film comprises three and five layer nonoriented structures of polyethylenes and polyesters or copolyesters.
U.S. Pat. No. 4,965,108 "Low Temperature Impact and Puncture Resistant Thermoplastic Films and Bags Therefrom," issued Oct. 23, 1990 to E. Biel et al. discloses multilayer film and bag structures comprising a polypropylene copolymer inner layer, an outer layer (e.g., a polyester or a polyamide), and a polypropylene based bonding resin therebetween.
U.S. Pat. No. 4,636,442 "Laminated Structures of Polyethylene Terephthalate and Elastomeric Copolyesterethers," issued Jan. 13, 1987, to R. Beavers et al. discloses multilayer films reportedly having improved flex-crack resistance. The films are based on polyethylene terephthalate and elastomeric copolyesterethers. Biaxially oriented three and five layer films in which the amount of copolyesterether is from about 5 to about 75 weight % (preferably 10 to 60 weight %) are disclosed.
U.S. Pat. No. 4,939,009 "Multilayer Sheets Having Excellent Adhesion," issued Jul. 3, 1990, also to R. Beavers et al., discloses three and five layer films based on polyolefins and copolyesterethers with tie layers therebetween.
U.S. Pat. No. 4,729,927 "Polyester Packaging Material," issued Mar. 8, 1988, to M. Hirose et al. discloses a packaging material comprising polyethylene terephthalate and a second material based on polyethylene isophthalate copolymerized with an aliphatic hydroxycarboxylic acid having up to eight carbon atoms. Reportedly, the number of layers is not particularly critical, although films with up to five layers are said to be preferred.
Japanese Kokai Patent No. 2-270553 "Multilayer Plastic Sheet with Gas Barrier Feature," published Nov. 5, 1990 discloses multilayer films based on layers of saponified ethylene/vinyl acetate copolymers, modified polyolefin adhesives, and thermoplastic polyesters.
Impact resistant and/or shatterproof security films for windows are also known. For example, U.S. Pat. No. 3,899,621 "Security Film for Shatter-Proofing Windows," issued Aug. 12, 1975 to M. Willdorf discloses three and five layer films comprising layers of polyesters and polyurethanes. Preferably, the polyester layers range in thickness from 0.5 to 5 mils and the polyurethane layers range in thickness from 0.2 to 0.4 mil. U.S. Pat. No. 3,891,486 "Process for Producing Solar Control Window," issued Jun. 24, 1975, also to M. Willdorf, discloses a solar control film comprising a pair of polyester (e.g., polyethylene terephthalate) layers each from 0.25 to 1 mil thick with a vapor-deposited aluminum coating and an adhesive therebetween.
U.S. Pat. No. 4,945,002 "Impact-Resisting Anti-Lacerative Window Units," issued Jul. 31, 1990 to I. Tanuma et al. discloses a three layer film comprising two exterior layers (e.g., an ethylene/vinyl acetate copolymer, an ethylene/vinyl acetate/triallyl isocyanurate terpolymer, a polybutyl butyral, a polyvinylformal, or a polyurethane), and an intermediate layer (e.g. polyethylene terephthalate, polyamides, polyester polyethers, polysulfones or polyimides) therebetween. The film is sandwiched between a pair of transparent glass or plastic plates.
Various tapes are also known. For example, U.S. Pat. No. 4,091,150 "Coextruded Polyester Splicing Tape," issued May 23, 1978 to G. Roelofs discloses a multilayer tape comprising a support film formed from a tough, flexible polyester (e.g. polyethylene naphthalate or polyethylene terephthalate) which is coextruded with an adhesion promoting polyester. A thermoset adhesive is adherently bonded to the adhesion promoting polyester.
U.S. Pat. No. 4,908,278 "Severable Multilayer Thermoplastic Film," issued Mar. 13, 1990 to Bland et al. discloses a multilayer film which may be easily and precisely cut in a straight line. The film comprises alternating layers of brittle and ductile materials. Japanese Kokoku Patent Publication No. 63-5394 "Laminate Film," published Oct. 26, 1988, discloses three and five layer tape backing films comprising layers of different polyesters. Reportedly, the films have good manual tearing properties.
U.S. Pat. No. 4,540,623 "Coextruded Multi-layer Articles," issued Sep. 10, 1985 to J. Im et al. discloses an impact resistant multilayer laminate comprising alternating layers (preferably at least about 40 layers) of coextruded polymeric thermoplastics wherein one of the materials contains a carbonate polymer. Suggested uses include glazing applications for windows and signs.
European Patent Application No. 0,426,636 "Iridescent Film with Thermoplastic Elastomeric Components," published May 8, 1991 discloses a transparent thermoplastic film of at least ten layers. The adjacent layers differ in refractive index and at least one of the layers is based on a thermoplastic elastomer resin. The layers range in thickness from 30 to 500 nanometers.